It has been known to replenish sand to eroded beaches to restore such beaches to their original sand profile before the erosion began. There has been a number of methods that have been attempted to prevent the erosion of nourishment sand. A known method has been to construct a "perch" to retain the nourishment sand. This perch may be formed from rocks or some other type of submerged offshore breakwater. One of the problems that has arisen is that when conventional submerged breakwaters have been used as a perch to prevent the loss of nourishment sand, they failed in this endeavor. Therefore, after a relatively short period of time all of the nourishment sand will have washed away and the beach will be back to its original sand profile before nourishment.
FIGS. 1A, 1B, 1C, and 1D graphically show the progression of erosion of nourishment sand when a conventional breakwater structure is used. Referring to FIG. 1A, this figure depicts the initial situation just after nourishment sand has been filled behind a conventional breakwater to raise the sand profile. At this time, there is no erosion of the nourishment sand behind the breakwater. Beach replenishment in this fashion is cost effective only as long as the nourishment sand is retained for a reasonable period of time.
Referring to FIG. 1B, as each wave passes over the submerged breakwater, an eddy is formed behind the breakwater. This eddy scours nourishment sand in this area behind the breakwater. The nourishment sand that is scoured by the eddy is transported from behind the breakwater by the wave backwash. This backwash transports the scoured nourishment sand seaward over the top of the breakwater.
Referring to FIG. 1C, the wave eddy and backwash cause progressive erosion in the landward direction of the nourishment sand behind the breakwater. The wave eddy and backwash also progressively scour deeper down behind the breakwater at the beachward face of the breakwater. The nourishment sand that is scoured from behind the breakwater due to the progressive erosion is transported by the combined eddy/backwash action to the seaward side of the breakwater. This sand may be deposited on, or seaward of, the seaward face of the breakwater. The depositing of this sand, as shown in FIG. 1C, results in reducing the efficiency of the breakwater.
Referring to FIG. 1D, the final equilibrium sand profile is shown for the situation when a conventional breakwater is used as a perch. As is shown in FIG. 1D, more than half of the nourishment sand has been lost due to the eddy/backwash action scouring nourishment sand from behind the breakwater. This scouring and nourishment sand loss extends all the way landward to the water's edge.
In a typical situation, this equilibrium condition will be reached in the first year after the nourishment sand has been filled behind the breakwater. Even though the equilibrium condition in FIG. 1D is reached, in four to eight years, even the remaining nourishment sand will be eroded away and only the original sand profile will be left.
Accordingly, there is a need for a submerged breakwater that may be used as a perch that will greatly reduce the erosion caused by the combined eddy/backwash action of waves passing over the submerged breakwater, thereby retaining more of the nourishment sand for a much longer period of time.